An optical disc, approximately 64 mm in diameter, having a recording capacity capable of recording music sound signals for 74 minutes or longer, is currently in use. This small-sized optical disc, termed a Mini-Disc (registered trademark), is classified into a replay-only disc, having data recorded as pits, and a recording and/or reproducing disc, having data recorded by a magneto-optical recording (MO) system and which may thus also be reproducible. The following description is directed to a small-sized recording and/or reproducing disc, referred to below as an optical disc. With this optical disc, the track pitch, the recording wavelength of the recording laser light or the NA of the objective lens have come to be ameliorated in order to increase disc's recording capacity.
In the following explanation, an optical disc of an initial stage, in which groove recording is carried out with the track pitch of 1.6 μm, is termed the first generation MD. The physical format of this first generation MD is prescribed as follows: The track pitch is 1.6 μm and the bit length is 0.59 μm/bit. The laser wavelength λ is set to λ=780 nm and the numerical aperture of the optical head NA is set to NA=0.45. The recording system employed is the groove recording system in which a groove (i.e. a groove formed on the disc surface) is used as a track for recording and/or reproduction. The address system employed is a system employing the wobbled groove in which a single-spiral groove is formed on a disc surface and in which a wobble as the address information is formed on both sides of this groove. Meanwhile, in the present specification, the absolute address recorded by the wobbling is termed an ADIP (Address in Pre-Groove).
In the conventional Mini-Disc, an EFM (8 to 14 modulation) system is employed as the recording data modulating system. As the error correction system, ACIRC (Advanced Cross Interleave Reed-Solomon Code) is used. For data interleaving, a convolution type data interleaving is used. In this manner, data redundancy amounts to 46.3%.
In the first generation MD, the data detection system is a bit-by-bit system, while the disc driving system used is the CLV (Constant Linear Velocity) system. The linear velocity of the CLV system is 1.2 m/sec.
The standard data rate during recording and/or reproduction is 133 kB/sec, while the recording capacity is 164 MB (140 MB for MD-DATA). The minimum data re-write unit (cluster) is constructed by 36 sectors composed of 32 main sectors and four link sectors.
In these days, the next-generation MD, having a recording capacity further improved over the first generation MD, is being developed. Such an MD in which the medium is unchanged from the conventional medium (disc orb cartridge), and in which the modulation system or the logical structure is changed to provide for a double-density user area to increase the recording capacity to for example 300 MB is now contemplated. This MD is referred to below as the next-generation MDI. The physical parameters of the recording medium are the same, the track pitch is 1.6 μm, the laser light wavelength λ is such that λ=780 nm and the numerical aperture of the optical head NA is such that NA=0.45. The recording system used is the groove recording system. The address system used is the ADIP. Thus, the structure of the optical system, ADIP address readout system and the servo processing in the disc driving device are similar to those of the conventional mini-disc.
An MD further improved in the recording capacity over the next-generation MD1 (next-generation MD2) is also being developed, in which the track pitch is reduced to 1.25 μm and in which a recording mark is detected by DWDD (Domain Wall Displacement Detection) from the aforementioned groove.
Meanwhile, if it is attempted to rotationally drive and thereby reproduce the next-generation MD2, in which the recording capacity has been increased by exploiting the DWDD, with the CLV (Constant Linear Velocity) in the same way as in reproducing the next generation MD1, the adverse effect ascribable to tracking offset becomes significant because the reproducing spot is larger than the mark. Specifically, if even the smallest tracking offset is produced, the result is that a mark from a neighboring track is also picked up, because of the narrow track pitch, thus significantly lowering the readout characteristics.
That is, with an optical disc, such as the next-generation MD2, which has the track pitch further narrowed and which is reproduced with ultra-high resolution by DWDD, it is necessary to cope with detracking extremely rigorously.
The next generation MD2 is also of the ADIP addressing system and, if the carrier frequency of the track is offset every track period by CLV, as shown in FIG. 1, the ADIP phase is also offset. With the next generation MD2, as in other MDs, the push-pull signal PP, detected with one spot, is detected and used as a tracking error signal. However, low frequency components of a few Hz appears as a beat component in the push-pull signal, as shown in FIG. 2. This push-pull signal is increased as a wobble push-pull signal WPP to the extent shown in FIG. 3. With the signal of the magnitude as large as that shown in FIG. 3, detracking is unavoidably produced in the next generation MD2.